1. Field of the Invention
The present disclosure relates to active pixel sensors, and more particularly to Complementary Metal-Oxide Semiconductor (CMOS) active pixel sensors.
2. Description of the Related Art
An Active Pixel Sensor (APS) converts a photo image into an electrical signal, and is widely used in digital cameras, mobile phones having a camera, vision systems, and the like.
Active pixel sensors are classified into a Charge-Coupled Device (CCD) type and a Complementary Metal-Oxide Semiconductor (CMOS) type. The CCD type has less noise than the CMOS type and has better image quality than the CMOS type. In addition, the CCD type has disadvantages, with respect to manufacturing costs and power consumption, compared with the CMOS type. The CMOS type can be manufactured using a semiconductor manufacturing process, so that the CMOS type can be easily integrated into peripheral systems having an amplifying block and a signal-processing block. The CMOS type has reduced manufacturing costs, a higher processing speed and reduced power consumption compared with the CCD type.
A CMOS active pixel sensor generally has a 3-transistor configuration and a 4-transistor configuration. In the 4-transistor configuration, a CMOS active pixel sensor includes one photo diode and four MOS transistors. Here, the photo-generated charge integrated at the photo diode is sensed under the control of the four MOS transistors. In the 3-transistor configuration, a CMOS active pixel sensor includes one photo diode and three MOS transistors. Here, the photo-generated charge integrated at the photo diode is sensed under the control of the three-MOS transistors.
FIG. 1 is a circuit diagram illustrating a conventional CMOS active pixel sensor having the 4-transistor configuration. Referring to FIG. 1, the CMOS active pixel sensor 100 having the 4-transistor configuration includes a photo diode PD, a transmission transistor M11, a reset transistor M12, a source follower transistor M13 and a selection transistor M14.
When a gate voltage RG of the reset transistor M12 increases and the reset transistor M12 is turned on, a potential of a sensing node, i.e., a floating diffusion node FD, increases up to a level of a power supply voltage VDD. The potential of the floating diffusion node FD is sampled by the source follower transistor M13 and the selection transistor M14, and the sampled potential is a reference potential.
When light is incident onto the photo diode PD during a photo integration period, electron hole pairs (EHP) are generated in response to the incident light. The charges integrated (or stored) in the photo diode PD are transmitted to the floating diffusion node FD when a gate voltage TG of the transmission transistor M11 increases after the photo integration period. A potential of a source of the source follower transistor M13 is changed when the potential of the floating diffusion node FD decreases substantially in proportion to the quantity of the transmitted charge.
The selection transistor M14 is turned on when a gate voltage SEL of the selection transistor M14 increases, a source voltage of the source follower transistor M13 is outputted as an output voltage VOUT. A photo-sensing is accomplished by measuring the difference between the reference potential and the output voltage VOUT. Then, the reset operation is repeated.
FIG. 2 is a circuit diagram illustrating a conventional CMOS active pixel sensor having the 3-transistor configuration. Referring to FIG. 2, the CMOS active pixel sensor 200 having the 3-transistor configuration includes a photo diode PD, a transmission transistor M21, a reset transistor M22, and a source follower transistor M23.
The CMOS active pixel sensor 200 having the 3-transistor configuration may not include the transmission transistor M11 of FIG. 1. In addition, the CMOS active pixel sensor 200 having the 3-transistor configuration does not include the selection transistor M14 of FIG. 1 and uses a dynamic power supply voltage DVD.
The dynamic power supply voltage DVD is increased to a high power supply voltage when the potential of the floating diffusion node FD is reset and is sensed, and otherwise dynamic power supply voltage DVD maintains a low power supply voltage. The function of the selection transistor is performed by the dynamic power supply voltage DVD.
When a gate voltage RG of the reset transistor M22 increases and the reset transistor M22 is turned on while the dynamic power supply voltage DVD has the high power supply voltage, a potential of a sensing node, i.e., a floating diffusion node FD, increases up to a level of the high power supply voltage. The potential of the floating diffusion node FD is sampled by the source follower transistor M23 and the sampled potential is outputted to an internal circuit. The sampled potential is a reference potential. Afterwards, the dynamic power supply voltage DVD decreases to a low power supply voltage.
When light is incident onto the photo diode PD during the photo integration period, electron hole pairs (EHP) are generated in response to the incident light. The charges integrated (or stored) in the photo diode PD are transmitted to the floating diffusion node FD when a gate voltage TG of the transmission transistor M21 increases after the photo integration period. When a potential of the floating diffusion node FD decreases substantially in proportion to the quantity of the transmitted charges while the dynamic power supply voltage DVD has the high power supply voltage, a source voltage of the source follower transistor M23 is changed. The source voltage of the source follower transistor M23 is outputted as the output voltage VOUT to the internal circuit. Then, the reset operation is repeated as explained in FIG. 1.
The active pixel sensor senses a signal based on the potential variation of the floating diffusion node FD. Namely, the photo-sensing is accomplished by measuring the difference between the reference potential and the output voltage VOUT.
A resolution is an important factor in digital cameras and mobile phones having a digital camera. Thus, pixel size is an important design factor. In addition, as the pixel size decreases, it is difficult to maintain the image quality of the pixel sensor beyond a predetermined image quality. In particular, the CMOS active pixel sensor is restricted to an optically symmetrical structure due to the characteristics of the CMOS active pixel sensor and the decreasing sizes of the transistors in the pixel, so that the CMOS active pixel sensor is very vulnerable to noise.